The global epidemiology of many infectious diseases has changed in recent years; a number of concomitant and mostly anthropogenic factors play a role in this process, including climate change, increased urbanisation, population mobility, deforestation, agricultural intensification and forced displacement of human populations. Most research has focused on malaria and other vector-borne infections or on and food- and waterborne diseases. The question of whether the timing of seasonal influenza epidemics has changed in recent years has received comparatively little attention. However, influenza seasonality is known to be linked to many of the above factors, and the temporal characteristics of influenza epidemics may evolve over time as a result of changes in these factors.

Seasonal influenza epidemics in the northern hemisphere are typically characterised by a short epidemic period of 8 to 12 weeks that varies in intensity during the winter months (November to March) and are associated with substantial morbidity and mortality. Annual vaccination is the most effective measure to reduce the burden of influenza and is most effective when vaccination campaigns coincide optimally with seasonal epidemics. Considering that 2 to 4 weeks may be required to develop an immune response to the vaccine and protection may wane within 6 months, the timing of the epidemic peak is an important element that should be considered to optimise the effectiveness of influenza vaccination campaigns.

To assess whether the epidemiology of influenza has changed in the World Health Organization (WHO) European Region (900 million inhabitants), we examined for each country and the Region as a whole whether the timing of epidemic peaks has changed (i.e. occurs earlier or later) between 1996 and 2016.